Lol! DH and I are experienced DIY homeowners. We recently bought a rental that needed "only" cosmetic updating. We were not at the inspection. Yes, that was against our better judgement, but it couldn't be helped, because the house is 7 hours by car from our primary residence. (Don't worry, we have another house in the same development and we'll spend at least half our time there when DH fully retires. Prices are down because of Canada and oil and all, so we got a screaming deal on the place.)
After closing, we worked like crazy on the house, and only when we were doing the final clean and polish did either of us actually step into the master shower. Yup, there's a problem, kinda like yours, except our shower pan is on the ground floor, on concrete. What's our solution? We're going point it out to prospective tenants and require that they refrain from showering in high-heeled shoes. If/when the pan starts to fail, we'll fix or replace it then. End of story. Not going to hassle with finger-pointing, because it's just not worth our time.
So OP, my advice is to take soccerluvof4's suggestion and then get on with your awesome life.